Evaluating historical, basin-wide landslide activity in a context of land abandonment and climate change: Effects of landslide visibility and temporal resolution

Drainage basins of the Northern Apennines, particularly in the clayey settings, bear among the highest rates of landsliding worldwide. A history of major land cover changes has left a landscape characterized by sparse, coppice-managed forest, transitional shrubs, and actively eroding badlands. Historical trends of landslide occurrence are examined in the Sillaro River basin (139 km2) in relation to land cover and climatic changes. To this purpose we have compiled a multi-temporal (1954–2018) landslide inventory (n = 1164) across twelve sequential photo sets that bears decadal (7- to 15-yr) and finer (2- to 6-yr) temporal resolution respectively before and after 1996. To account for changes in meteorological forcing, we examine: (i) the total annual precipitation (PRCPTOT); (ii) the annual maximum daily precipitation (RX1day); and (iii) the precipitation fraction (R99pTOT) due to extremely wet days. We find that landslide activity is strongly controlled by lithology, with landslide densities in claystones 3-to-4 times higher than in marl-sandstone alternations. This difference is chiefly associated with badlands, which are the most active land cover type and where new scars at a site could recur up to nine times. To evaluate the influence of varying temporal resolution on inventory completeness, hence on inference about land cover and climatic effects, we constrain the time scales of landslide visibility and assess the relative rates of undersampling. We find that visibility functions decline non-linearly with time, and that an inventory compiled at 5-year resolution would be missing up to 20 % of the landslide scars, with the size of an additional 27 % that would be underestimated due to revegetation. Overall, detection of entire landslide scars, which varies with land cover, becomes rare after 13 years in transitional shrubs, and after 17 years in badlands and managed forest. The historical analysis shows that landslide count: (i) increases in 1955–1976, a period of maximum anthropogenic pressure and wetter conditions; (ii) decreases steadily from 1977 through 2000, during a phase of land abandonment and decline in annual precipitation; and (iii) grows highest in 2000–2014, a period of land cover stability characterized by lesser precipitation although increasingly focused on high-magnitude events. To evaluate the likely reason of this recent increase in landsliding (i.e., R99pTOT vs inventorying resolution), we replicate the post-1996 mapping at coarser resolution. In the simplified inventory, landslide densities drop up to a factor of 2, and the inverse correlation originally linking landslide count with R99pTOT, loses significance. We conclude that, when the bias associated with varying inventorying resolution is removed, dependencies previously attributed to climatic effects become drastically reduced, and in some instances can even disappear

Controls over particle motion and resting times of coarse bed load transport in a glacier‐fed mountain stream

Coarse bed load transport is a crucial process in river morphodynamics but is difficult to monitor in mountain streams. Here we present a new sediment transport dataset obtained from two years of field‐based monitoring (2014‐2015) at the Estero Morales, a high‐gradient stream in the central Chilean Andes. This stream features step‐pool bed geometry and a glacier‐fed hydrologic regime characterized by abrupt daily fluctuations in discharge. Bed load was monitored directly using Bunte samplers and by surveying the mobility of PIT (passive integrated transponder) tags. We used the competence method to quantify the effective slope, which is the fraction of the topographical slope responsible for bed load transport. This accounts for only 10% of the topographical slope, confirming that most of the energy is dissipated on macroroughness elements. We used the displacement lengths of PIT tags to analyze displacement lengths and virtual velocity of a wide range of tracer sizes (38‐415 mm). Bed load transport in the Estero Morales show to be size‐selective and the distance between steps influences the displacement lengths of PIT tags. Displacement lengths were also used to derive the statistics of flight distances and resting times. Our results show that the average length of flight scales inversely to grain size. This contradicts Einstein's conjecture about the linear relationship between grain size and intervals between resting periods in a steep step‐pool stream in ordinary flood conditions

Variable hillslope-channel coupling and channel characteristics of forested mountain streams in glaciated landscapes

Channel morphology of forested, mountain streams in glaciated landscapes is regulated by a complex suite of processes, and remains difficult to predict. Here, we analyze models of channel geometry against a comprehensive field dataset collected in two previously glaciated basins in Haida Gwaii, B.C., to explore the influence of variable hillslope\u2013channel coupling imposed by the glacial legacy on channel form. Our objective is to better understand the relation between hillslope\u2013channel coupling and stream character within glaciated basins. We find that the glacial legacy on landscape structure is characterized by relatively large spatial variation in hillslope\u2013channel coupling. Spatial differences in coupling influence the frequency and magnitude of coarse sediment and woody material delivery to the channel network. Analyses using a model for channel gradient and multiple models for width and depth show that hillslope\u2013channel coupling and high wood loading induce deviations from standard downstream predictions for all three variables in the study basins. Examination of model residuals using Boosted Regression Trees and nine additional channel variables indicates that ~10 to ~40% of residual variance can be explained by logjam variables, ~15\u201340% by the degree of hillslope\u2013channel coupling, and 10\u201320% by proximity to slope failures. These results indicate that channel classification systems incorporating hillslope\u2013channel coupling, and, indirectly, the catchment glacial legacy, may present a more complete understanding of mountain channels. From these results, we propose a conceptual framework which describes the linkages between landscape history, hillslope\u2013channel coupling, and channel form. \ua9 2018 John Wiley & Sons, Ltd

Bedload transport in a formerly glaciated mountain catchment constrained by particle tracking

In formerly glaciated mountain settings, Pleistocene glaciations are responsible for profound spatial reorganization of the landscape structure. By imposing local channel slope and the degree of hillslope-channel connectivity, glacial macro-forms can exert first-order controls on the downstream strength and continuity of the coarse sediment cascade. To estimate quantitatively these controls we trace bedload transport for 3 years along Strimm Creek, Eastern Italian Alps. Specifically, we monitor the travel distance of 490 PIT-tagged particles (b axis: 23-229 mm; weight: 83-6525 g) at two contrasting sites: Upper Strimm Creek (US; 4 km2), which flows through a fluvially dominated hanging valley, and Lower Strimm Creek (LS; 7.5 km2), located downstream, in a relict glacial trough where it experiences periodic colluvial sediment inputs from lateral tributaries. Tracer positioning within the streambed is periodically tracked in the field with a portable antenna in order to assess progressive travel distances, as well as the extent of the channel active layer, in relation to snowmelt and rainfall-driven peak flows. Interestingly, we show that tracer virtual velocities for selected inter-survey periods are independent of tracer weight at both study sites. Cumulatively, tracers in US have travelled across distances (i.e. inner quartiles) shorter than 2 m, which correspond to over 2 orders of magnitude less than what was observed in LS. These figures translate, after calculations of tracer inter-survey virtual velocities, into estimated bedload volumes equal to about 3 m3 in US and 600 m3 in LS, with most of the transport (75 % in US, and 93 % in LS) occurring during snowmelt. A similar contrast in bedload transport rates, even without considering the additional volumes of material mobilized by mass-wasting processes in LS, testifies the extent to which the glacial imprinting can still affect contemporary sediment transfer, and thus postglacial landscape evolution, in mountain drainage basins

A novel topographic parameterization scheme indicates that martian gullies display the signature of liquid water

Martian gullies resemble gullies carved by water on Earth, yet are thought to have formed in an extremely cold (2-driven processes. That this argument persists demonstrates the limitations of morphological interpretations made from 2D images, especially when similar-looking landforms can form by very different processes. To overcome this we have devised a parameterization scheme, based on statistical discriminant analysis and hydrological terrain analysis of meter-scale digital topography data, which can distinguish between dry and wet surface processes acting on a landscape. Applying this approach to new meter-scale topographic datasets of Earth, the Moon and Mars, we demonstrate that martian gullied slopes are dissimilar to dry, gullied slopes on Earth and the Moon, but are similar to both terrestrial debris flows and fluvial gullies. We conclude that liquid water was integral to the process by which martian gullies formed. Finally, our work shows that quantitative 3D analyses of landscape have great potential as a tool in planetary science, enabling remote assessment of processes acting on planetary surfaces

Characterization of a debris flow event using an affordable monitoring system

This study presents monitoring data of a debris flow event in the Central Italian Alps. The debris flow occurred on August 16, 2021 in the Blè basin (Val Camonica valley, Lombardia Region) and was recorded by a monitoring station installed just few weeks before. The monitoring system was deployed to document the hydrologic response of the catchment to rainfall, and was designed to be lightweight, relatively cheap, and easy to deploy in the field. To this purpose, we combined video cameras with geophysical sensors (geophones and infrasound) and optimized the power supply system. The data recorded during the event allowed to identify the triggering rainfall, document the flow behaviour, and estimate surface flow velocity and flow rate using Particle Image Velocimetry algorithms. Moreover, the seismic signal generated by the debris flow revealed a peculiar frequency spectrum compared to regular streamflow. These results show that even a relatively simple monitoring system may provide valuable data on real debris flow events